For those who love the twelve souls cut down in the Washington DC navy yard there will never be satisfying answer to why it all happened. But there is good reason for all of us to wonder how -- -- could walk into a government building with a sawed off shotgun. And constructively moved from floor -- floor for an hour before police could take him down. New surveillance video of air and Alexis offer some cautionary clues. And ABC's Pierre Thomas -- -- This is -- Alexis armed with a sawed off shotgun hunting innocent victims. Seen here for the first time in new and chilling FBI video. It started as a routine Monday at the Washington navy yard. Alexis shows -- pass in his waved in by a security guard just before 8 AM. Surveillance cameras captured him driving into a parking garage and moments later walking in through the front door building 197. Stop -- The sawed off shotgun might be in the backpack you see right here. But there's no metal detector no -- check. Alexis is supposed to be the year he's been hard as a civilian contractor. Working on the building's computer system. According to the FBI he first goes to a men's bathroom on the fourth floor. There they believed he readies himself with a Rampage leaving his bag behind. We next see Alexis armed with a shotgun and checking doors peeking around corners looking for someone anyone to ambush. Slow down the tape. Look at how methodical he is calm yet urgent trying to catch someone unsuspecting. Watch closely he sees some people and ducks behind a wall. When you look again you can see the intent -- stalking. And over the course of approximately one hour shoots and kills a dozen victims wounding four more. And today the first official explanation. On what drove a Lexus to kill mr. Alexis was -- mentally unstable individual and he was able. To obtain a -- the shotgun is the primary weapon on -- he adds a number of carvings including end to the torment. Better off this way. And -- ELF weapons Alexis had been hearing voices and believed he was being bombarded by microwaves. Something naval authorities knew about. There are multiple indicators that Alexis held a delusional belief. He was being controlled or influenced by extremely low frequency. Porgy Al. Electro magnetic waves. Ended his computer files the FBI found a perhaps telling message. -- true low frequency attack is what I've been subject to for the last three months and to be perfectly honest that is what is driven me to this -- the system breakdown. He had various. Encounters a law enforcement across the country. Various. Time -- what doctors as well I can't comment on exactly where it may have broken down. Red flags were clearly missed -- should have been some kind of mental health assessment done. In order to determine. The severity of these hallucinations. There's no indication that was done ninety minutes after Lexus if you're building 197. Police shot him dead. There are indicators that allow access accepted death as the inevitable consequence of -- actions were Nightline I'm Pierre Thomas in Washington.

This transcript has been automatically generated and may not be 100% accurate.